Site-selective nanostructure growth and material incorporation at the atomic scale offer a promising pathway for engineering quantum materials and nanodevices. Here, GaAs nanowires (NWs) with an axial heterostructure of alternating zinc blende (Zb) and wurtzite (Wz) crystal phases are employed as templates for site-selective Ga and Bi overgrowth. Using X-ray photoemission electron microscopy (XPEEM) with nanoscale spatial resolution, we map elemental distribution and local chemical bonding to reveal the incorporation behavior of Bi atoms in 110 Zb and 11-20 Wz facets. Bi incorporation proceeds through an anion-exchange process, where Bi atoms replace As, forming local Ga-Bi bonds and producing a thin GaAs1-xBix shell. We observe crystal-phase-dependent Bi incorporation, with higher Bi concentration in the Zb segments than in the neighboring Wz segments within the same NW. Furthermore, the Zb segment with higher Bi content exhibits reduced susceptibility to oxidation compared with the Wz segment, resulting in increased Ga-oxide in the Wz surfaces. This study highlights GaAs NW Zb/Wz heterostructures as a template for controlled growth of GaBi and GaAs1-xBix nanostructures with tailored functionalities for quantum applications

Atomic layer deposition (ALD) of HfO x layers has emerged as a promising strategy for interface passivation in chalcopyrite-based thin-film solar cells. However, the nucleation dynamics of (Ag,Cu)(In,Ga)Se2 (ACIGS) absorbers remain insufficiently understood, particularly regarding the interplay between absorber composition and alkali postdeposition treatments (PDTs). Here, we employ in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the initial surface chemistry during HfO x ALD on ACIGS thin-film solar cell absorbers with high [Ga]/([Ga]+[In]) (GGI) ratio subjected to RbF-PDT and compare the findings with low-GGI counterparts (Martin et al., ACS Appl. Energy Mater. 2025, 8, 461-472). The results show that high-GGI ACIGS surfaces are strongly Cu-depleted and enriched in Se and alkali-metal-containing secondary phases, which hinders tetrakisdimethylamido-hafnium (TDMA-Hf) precursor adsorption and delays oxide nucleation, in contrast to low-GGI ACIGS that allow quicker and more efficient HfO x growth. Water pulses are identified as essential for reactivating the RbF-treated high-GGI surface by generating hydroxyl groups, thereby enabling metal precursor activation. The distinct formation of Ga-F and In-O species is only observed on high-GGI ACIGS together with increased Na diffusion, reflecting different surface chemistry for a higher Ga content of ACIGS as compared to the low Ga case. These findings demonstrate that bulk composition and alkali-PDT strongly influence ALD reactivity, surface passivation, and interface formation, with direct implications for composition-specific optimization of ACIGS solar cells.

Heavily doped perovskite oxide thin films have emerged as versatile catalysts based on exsolution processes, which yield functional nanoparticles, but also introduce dopant inhomogeneities and internal phase separation. Here, we resolve the initial-stage of thin film growth in heavily Ni-doped and (Nb,Ni)-co-doped SrTiO3-δ utilizing scanning tunneling microscopy and spectroscopy (STM/S). While the growth of co-doped SrTiO3-δ follows statistically distributed island nucleation, Ni-doped SrTiO3-δ (STNi) exhibits a distinct pentagonal or hexagonal pattern of monolayer islands. A similar pattern is observed in thicker films by transmission electron microscopy (TEM). STS reveals Ni-dopant clusters forming from the onset of the thin film growth and serving as nucleation points for embedded nanostructures at increasing film thickness. Quantitative analysis reveals that in the as-grown monolayers, approximately 20% of the Ni ions segregate into nanoclusters, while the remaining ∼80% are dissolved in the perovskite lattice. Upon reduction (600°C, UHV), the entire Ni content of the sub-monolayer STNi is found to nucleate as metallic nanoparticles at the surface, revealing two exsolution pathways from the perovskite lattice as well as from pre-defined clusters. These findings provide deeper insights into the nucleation processes in heavily doped oxide and further reveal the role of dopant inhomogeneities for metal exsolution reactions.

ABSTRACT Heavily doped perovskite oxide thin films have emerged as versatile catalysts based on exsolution processes, which yield functional nanoparticles, but also introduce dopant inhomogeneities and internal phase separation. Here, we resolve the initial‐stage of thin film growth in heavily Ni‐doped and (Nb,Ni)‐co‐doped SrTiO 3‐δ utilizing scanning tunneling microscopy and spectroscopy (STM/S). While the growth of co‐doped SrTiO 3‐δ follows statistically distributed island nucleation, Ni‐doped SrTiO 3‐δ (STNi) exhibits a distinct pentagonal or hexagonal pattern of monolayer islands. A similar pattern is observed in thicker films by transmission electron microscopy (TEM). STS reveals Ni‐dopant clusters forming from the onset of the thin film growth and serving as nucleation points for embedded nanostructures at increasing film thickness. Quantitative analysis reveals that in the as‐grown monolayers, approximately 20% of the Ni ions segregate into nanoclusters, while the remaining ∼80% are dissolved in the perovskite lattice. Upon reduction (600°C, UHV), the entire Ni content of the sub‐monolayer STNi is found to nucleate as metallic nanoparticles at the surface, revealing two exsolution pathways from the perovskite lattice as well as from pre‐defined clusters. These findings provide deeper insights into the nucleation processes in heavily doped oxide and further reveal the role of dopant inhomogeneities for metal exsolution reactions.

Abstract Bismuth-based compounds, such as Bi1-xSbx or Bi2Te3, have outstanding electronic properties especially for advanced quantum devices. However, the potential of low-dimensional group V-Bi materials is largely undetermined. Here, we report the experimental realization of a two-dimensional (2D) BiAs layer with giant Rashba spin splitting, grown on an InAs(111)B substrate via molecular beam epitaxy. ARPES reveals the emergence of a prominent M-shaped band structure and a distinct electron pocket at the Fermi level. DFT, complemented by synchrotron-based XPS, LEED, and STM, confirms strong spin-orbit coupling and a giant Rashba coefficient of these electronic states. An As overlayer, remaining from the fabrication process, preserves a structural shift between substrate and BiAs layer, essential for the Rashba splitting, while preventing undesirable reconstruction. This indicates how well-known capping techniques can stabilize new 2D compounds with interesting electronic structures, especially BiAs, a promising candidate to open possibilities for next-generation spintronic devices and field-effect transistors.

Nanostructured materials play a key role in modern technologies adding new functionalities and improving the performance of current and future applications. Due to their nature resulting in diffused heterogeneous structures (chemical and electronic composition typically organized in phases or building blocks) characterizing these materials needs state of the art technologies which combine nanometer spatial resolution, environmental reliability, and operando capabilities. Scanning photoelectron spectromicroscopy (SPEM) is one of the characterization tools that combine high spectral resolution X-ray photoelectron spectroscopy with submicron spatial resolution. In particular, the SPEM equipment hosted at the ESCA microscopy beamline at Elettra is capable of in situ and operando analysis regardless of sample morphology. The review presents three different case studies illustrating the capabilities of SPEM in the investigation of catalytic materials in different conditions and processes.

Investigating metal-semiconductor interfaces is crucial for enabling ultra-thin film growth and tuning the electronic properties. Here we have studied the atomic mechanism of Bi incorporation into the surface of InSb, the III-V semiconductor with the largest spin-orbit coupling, using surface science techniques and theoretical modeling. Bi deposition onto Sb-terminated InSb(111) surfaces at elevated sample temperature results in the formation of Bi monomers (alpha-Bi) and Bi trimers (beta-Bi), in a (2 x 2) and (3 x 3) superstructure, respectively. Ab initio calculations for various possible Bi-integration models confirm the atomic arrangement of alpha-Bi and beta-Bi superstructures and their energetically favorable formation. Both structures are based on strong covalent Bi-Sb bonds. This Bi-Sb compound is robust and has self-limiting thickness, which does not increase upon prolonged Bi deposition. On the contrary, Bi deposition at room temperature results in metallic Bi islands and layers of increasing thickness. However, the above-mentioned Bi-Sb compound is found unchanged in thickness after additional Bi deposition at room temperature and subsequent annealing, indicating the presence of a welldefined Bi-Sb interface even between InSb substrates and thicker metallic Bi surface layers. Tunable Bi surface structures and the self-limiting formation of a robust Bi-Sb interface offer a promising pathway towards developing Bi/InSb-based devices that combine a very large spin-orbit interaction with a technologically welldeveloped semiconductor platform, featuring superior electronic and optoelectronic properties.

Surface physics play an outsized role in nanostructured electronic devices such as solar cells. Semiconductor nanowires are perfect candidates for advanced solar cells due to their outstanding light absorption properties and their flexibility in axially stacking materials of different doping and band gap. Due to nanowire geometry, however, their surfaces dominate device performance and at the same time are challenging to investigate. Kelvin probe force microscopy (KPFM), an atomic force microscopy (AFM)-based method, provides a unique structural and electrical characterization even in unconventional 3D geometries. We demonstrate a high-resolution, non-destructive AFM technique for directly measuring nanowires within an array and still on their growth substrate. This in situ approach ensures measurement integrity and relevance while preserving the structures for subsequent measurement and processing. When compared with electron beam-induced current, cross-sectional KPFM is both more surface sensitive and less destructive. Utilizing such a cross-sectional approach facilitates rapid and comprehensive characterization of nanoelectronic surfaces.

X-ray methods can offer unique insights into the structural and electronic properties of nanomaterials. Recent years have seen a dramatic improvement in both x-ray sources and x-ray optics, providing unprecedented resolution and sensitivity. These developments are particularly useful for nanowires, which are inherently small and give weak signals. This review gives an overview of how different x-ray methods have been used to analyze nanowires, showing the different types of insight that can be gained. The methods that are discussed include x-ray diffraction, x-ray fluorescence, x-ray photoelectron spectroscopy and x-ray photoelectron emission microscopy, as well as several others. The review is especially focused on high spatial resolution methods used at the single nanowire level, but it also covers ensemble experiments.

Nanoscale optoelectronic components achieve functionality via spatial variation in electronic structure induced by composition, defects, and dopants. To dynamically change the local band alignment and influence defect states, a scanning gate electrode is highly useful. However, this technique is rarely combined with photoexcitation by a controlled external light source. We explore a setup that combines several types of light excitation with high resolution scanning gate and atomic force microscopy (SGM/AFM). We apply the technique to InAs nanowires with an atomic scale defined InP segment, that have attracted considerable attention for studies of hot carrier devices. Using AFM we image the topography of the nanowire device. SGM measurements without light excitation show how current profiles can be influenced by local gating near the InP segment. Modelling of the tip and nanowire can well predict the results based on the axial band structure variation and an asymmetric tip. SGM studies including light excitation are then performed using both a white light LED and laser diodes at 515 and 780nm. Both negative and positive photoconductance can be observed and the combined effect of light excitation and local gating is observed. SGM can then be used to discriminate between effects related to the wire axial compositional structure and surface states. The setup explored in the current work has significant advantages to study optoelectronics at realistic conditions and with rapid turnover.

HfO2, one of the most common materials in resistive switching devices, can stabilize in a ferroelectric orthorhombic phase, enabling two nonvolatile polarization states via oxygen displacement in the unit cell. Under certain conditions, ferroelectric and resistive switching can coexist, independently addressable, within one device. This study employs operando spectroscopic analysis to elucidate the role of oxygen in both switching processes. A conductive filament is identified through a local valence change at the oxide surface via X-ray Photoelectron Emission Microscopy, allowing vacancy density and filament diameter evaluation. This provides well-founded experimental evidence of a conductive filament in orthorhombic Hf0.5Zr0.5O2-δ (HZO) in application-relevant device geometry. Depth-dependent changes in the electronic signature of HZO and La0.8Sr0.2MnO3-δ (LSMO) with ferroelectric field cycling are identified by Hard X-ray Photoelectron Spectroscopy. Polarization-dependent shifts in the Hf core level align with the oxygen vacancy migration during ferroelectric switching. Fatigue-related vacancy generation causes an inhomogeneous reduction that does not propagate into the bottom electrode and extended domain pinning at the HZO/LSMO interface. This highlights the importance of interface engineering for the ferroelectric performance and of the oxygen affinity of the bottom electrode for both switching regimes.

Low-dimensional topological states have transformed our understanding of charge transportation through quantum materials. Many relevant observations have been connected to bismuth (Bi) containing materials or ultrathin Bi films. Here, we studied sub-monolayer amounts of Bi deposition on the In-terminated InSb(111)A surface using various complementary surface science techniques. Bi deposition at elevated sample temperature results in well-ordered (2 x 2) and (2 root 3 x 2 root 3)-R30 degrees surface reconstructions. Scanning tunneling microscopy/spectroscopy (STM/S) data show an enhanced density of states at the interface of the two reconstructions and local Bi trimers over the (2 x 2) reconstructed surface. Bi-induced metallic surface states crossing the Fermi level are observed and attributed exclusively to the localized trimer states through STS and angle-resolved photoemission spectroscopy (ARPES) results. Furthermore, the ARPES spectra show band splitting at the P point and degenerate surface states at the M point, which is associated with Rashba splitting due to strong Bi-substrate interaction. Thus we interpret the Bi/InSb(111)A, exhibiting spin-split metallic surface states induced by localized Bi structures, as a promising candidate for exploring low-dimensional states and spin dynamics in future quantum materials.

Ambient pressure X-ray photoelectron spectroscopy is employed to study in real time the chemical reactions occurring on (Ag,Cu)(In,Ga)Se2 (ACIGSe) surfaces during the first atomic layer deposition (ALD) cycle of HfO x under realistic synthesis conditions by using tetrakisdimethylamido-hafnium (TDMA-Hf) and H2O precursors. We find that the initial deposition due to surface reactions of HfO x ALD on ACIGSe depends on the pretreatment of the ACIGSe surface. While the growth of HfO x occurs directly upon exposure to the metal precursor for the nontreated (i.e., as-deposited) ACIGSe surface through chemical reactions, the growth is slower for the ACIGSe surface pretreated by postdeposition treatment by RbF. In the latter case, the diffusion of alkali and fluorine elements at the surface is observed during the ALD growth, thus leaving less reactive sites for the TDMA-Hf molecules to adsorb on. The results indicate that an optimization of the ALD of HfO x on ACIGSe needs to be taken into consideration for alkali metal fluoride-treated ACIGSe.

Metal-halide perovskites (MHPs) have gained substantial interest in the energy and optoelectronics field. MHPs in nanostructure forms, such as nanocrystals and nanowires (NWs), have further expanded the horizons for perovskite nanodevices in geometry and properties. A partial anion exchange within the nanostructure, creating axial heterojunctions, has significantly augmented the potential applications. However, surface degradation and halide ion migration are deteriorating device performance. Quantitative analysis of halide metal concentration and mapping of the electrical surface potential along the operating NW device are needed to better understand ion transportation, band structure, and chemical states, which have not been experimentally reported yet. This requires a characterization approach that is capable to provide surface-sensitive chemical and electrical information at the sub mu m scale. Here, we used operando nanofocused X-ray photoelectron spectroscopy (nano-XPS) to study CsPbBr3/CsPb(Br1-x Cl x )3 heterojunction NW devices with a spatial resolution of 120 nm. We monitored Br- and Cl- ion migration and comprehended the potential drop along the device during operation. Ion migration and healing of defects and vacancies are found for applied voltages of as low as 1 V. We present a model delineating band bending along the device based on precise XPS peak positions. Notably, a reversible redox reaction of Pb was observed, that reveals the interaction of migrating halide ions, vacancies, and biased metal electrodes under electrical operation. We further demonstrate how X-ray-induced surface modification can be avoided, by limiting exposure times to less than 100 ms. The results facilitate the understanding of halide ion migration in MHP nanodevices under operation.

The two-dimensional electron gas (2DEG) that forms on a semiconductor surface can be used to explore a variety of phenomena in quantum physics and plays an important role in nanoscale electronics, such as transistors. Controlling its formation is, thus, relevant. Using angle-resolved photoemission spectroscopy (ARPES) and accumulating the signal over many nanocrystals, we find that on clean InAs nanosheets with non-polar surfaces and wurtzite (WZ) crystal structures, a 2DEG can be observed at the Γ-point. We suggest that the step morphology on the WZ InAs specimens facilitates the appearance of the electron gas, since previous studies on InAs nanowire surfaces with the same crystal facet and a similar defect density did not exhibit a 2DEG. Subsequently, bismuth deposition leads to the disappearance of the 2DEG as well as a shift of the valence band. This is in contrast to previous observations on InAs surfaces, in which metal deposition would lead to the formation of a 2DEG. The control of the 2DEG with the addition of Bi atoms is relevant for applications of InAs nanosheets in quantum technologies. This study also illustrates that ARPES accumulated over several 2D materials oriented randomly around their normal axis can provide valuable information on their band structure with a fast turnover and low irradiation.

Nonvolatile memory devices based on ferroelectric HfxZr1-xO2 (HZO) show great promise for back-end integrable storage and for neuromorphic accelerators, but their adoption is held back by the inability to scale down the HZO thickness without violating the strict thermal restrictions of the Si CMOS back end of line. In this work, we overcome this challenge and demonstrate the use of nanosecond pulsed laser annealing (NLA) to locally crystallize areas of an ultrathin (3.6 nm) HZO film into the ferroelectric orthorhombic phase. Meanwhile, the heat induced by the pulsed laser is confined to the layers above the Si, allowing for back-end compatible integration. We use a combination of electrical characterization, nanofocused scanning X-ray diffraction (nano-XRD), and synchrotron X-ray photoelectron spectroscopy (SXPS) to gain a comprehensive view of the change in material and interface properties by systematically varying both laser energy and the number of laser pulses on the same sample. We find that NLA can provide remanent polarization up to 2P(r)= 11.6 mu C/cm(2) in 3.6 nm HZO, albeit with a significant wake-up effect. The improved TiN/HZO interface observed by XPS explains why device endurance goes beyond 10(7) cycles, whereas an identical film processed by rapid thermal processing (RTP) breaks already after 10(6) cycles. All in all, NLA provides a promising approach to scale down the ferroelectric oxide thickness for emerging HZO ferroelectric devices, which is key for their integration in scaled process nodes.

Recent work showing a strong quality improvement of the Si/SiO 2 material system by low temperature atomic hydrogen annealing (AHA), and the fact that III–V semiconductors outperform Si in many applications makes the investigation of AHA on III–V/high-k interfaces to a very interesting topic. In this work, the potential of AHA as a low temperature annealing treatment of InGaAs metal–oxide–semiconductor field-effect transistors is presented and compared to conventional annealing in a rapid thermal process (RTP) system using forming gas. It is found that post metal annealing in atomic hydrogen greatly enhances the quality of the metal–oxide–semiconductor structure in terms of effective mobility, minimum subthreshold swing, and reliability. The device performance is comparable to RTP annealing but can be performed at a lower temperature, which opens up for integration of more temperature-sensitive materials in the device stack.

III-Nitride based light-emitting diodes based on InGaN active layers formed on GaN, are fine for the blue and green emitting LEDs. However, the large lattice mis-match between red-emitting active layers and the GaN substrate still limits the efficiencies to very low values, typically <5%. We propose to use seeding techniques originally developed for nanowire growth, to seed the formation of ternary InGaN pyramids which later are converted to thin c-facet platelets of InGaN. I will in this presentation show that such relaxed, and dislocation-free, InGaN platelets with In-composition about 20%, have the potential as ideal templates for red-emitting microLEDs. Of special significance is the fact that our technology provides all three RGB microLED sources with one and the same InGaN technology, and with the same efficiency and with the maintained efficiency down to pixel sizes of just 1-2 µm in diameter.

The chemical bonding at the interface between compound semiconductors and metals is central in determining electronic and optical properties. In this study, new opportunities for controlling this are presented for nanostructures. We investigate Bi adsorption on 2D wurtzite InAs (112̄0) nanosheets and find that temperature-controlled Bi incorporation in either anionic- or cationic-like bonding is possible in the easily accesible range between room temperature and 400 °C. This separation could not be achieved for ordinary zinc blende InAs(110) surfaces. As the crystal structures of the two surfaces have identical nearest neighbour configurations, this indicates that overall geometric differences can significantly alter the adsorption and incorporation. Ab initio theoretical modelling confirms observed adsorption results, but indicate that both the formation energies as well as kinetic barriers contributes to the observed temperature dependent behaviour. Further, we find that the Bi adsorption rate can differ by at least 2.5 times between the two InAs surfaces while being negligible for standard Si substrates under similar deposition conditions. This, in combination with the observed interface control, provides an excellent opportunity for tuneable Bi integration on 2D InAs nanostructures on standard Si substrates.

Understanding the reaction mechanisms involved during the early stage of atomic layer deposition (ALD) of HfO2 on InAs is a key requirement for improving interfaces in III-V semiconductor-based devices. InAs is an excellent candidate to outperform silicon regarding speed and power consumption, and combined with HfO2, it gives promise for a new generation of ultra-fast MOSFETs. However, an improved interface quality and in-depth understanding of the involved surface species are needed. Here, we use in situ and operando ambient pressure XPS to follow in real-time the reaction mechanisms which control the ALD chemistry. Besides the removal of all unwanted oxide from the III-V, the same oxygen atoms are found to form HfOx already from the first half-cycle. In contrast to the standard ALD model, no hydroxyl groups are needed on the InAs surface. Furthermore, we observe an insertion reaction forming unexpected surface species. The second ALD half-cycle allows the immediate removal of all organic species leaving behind a uniform HfO2 layer partially terminated by hydroxyl groups. We find that prolonged exposure times upon both half-cycles guarantee a sharp InAs/HfO2 interface. Such an improved interface is an important step towards fast and sustainable III-V semiconductor-based electronics.